1. Fields of the Invention
The present invention relates to an angular velocity sensor for detecting an angular velocity acting on a vehicle such as an automobile, airplane, or an electronic equipment. More particularly, the present invention relates to a resonance type angular velocity sensor which excites a mass portion corresponding to an inertia mass, and detects an angular velocity on the basis of a displacement of the mass portion due to a Coriolis force generated in a direction perpendicular to both directions of an exciting direction of the mass portion and a rotating axis direction of the angular velocity sensor.
2. Description of the Related Art
FIGS. 1A, 1B, and 1C show a structure of a resonance type vibrating component such used in as a conventional angular velocity sensor. In a vibrating component 150, a plate like mass portion 152 corresponding to an inertia mass is supported in the vertical direction in the drawing by respective two beams 151 having an end fixed to a frame portion 160 of a silicon substrate 520. Comb electrodes 156 are formed at the side portions in the horizontal direction in the drawing of the mass portion 152, and comb electrodes 158 are respectively formed in the frame portion 160 in such a manner as to mesh with and oppose the comb electrodes 156.
An exciting conductive layer (not shown) is connected to each of these comb electrodes 156 and 158. When an alternating voltage is applied to the exciting conductive layer, an electrostatic force is generated between the comb electrodes 156 and 158, and the mass portion 152 displaces to the lateral direction in the drawing due to the electrostatic force, to vibrate in the manner as shown in FIG. 1B.
During lateral excitation of the mass portion 152 in the above manner, when the angular velocity has a rotational axis in the horizontal direction of the drawing, the Coriolis force is generated in a direction perpendicular to both the exciting direction and the rotating axis direction. When the Coriolis force acts on the mass portion 152, the mass portion 152 displaces to the operating direction of the Coriolis force, the thickness direction of the mass portion 152 in FIG. 1A supported by beam 151, and vibrates.
FIG. 1C shows a cross section along a line 1C--1C in FIG. 1A, and, as shown in FIG. 1C, displacement detecting electrodes 155 and 157 for detecting a displacement to the thickness direction of the mass portion 152 are provided at positions so that the mass portion 152 is held therebetween from the thickness direction. When the mass portion 152 displaces to the thickness direction due to the Coriolis force, as mentioned above, the detecting electrodes 155 and 157 detect this displacement by, for example, a capacity detecting method or the like. Then, a signal in response to a magnitude of an amplitude of a vibration of the mass portion 152 due to the Coriolis force is obtained and the magnitude of the angular velocity or the like is detected.
In a conventional angular velocity sensor 150, the beam 151 supports the mass portion 152 in the exciting direction in such a manner as to freely move, and also supports the mass portion 152 in the Coriolis force generating direction in such a manner as to freely displace. Accordingly, the operating direction of the Coriolis force with respect to the mass portion 152 becomes the thickness direction of the substrate. Accordingly, in order to detect the displacement to the thickness direction of the mass portion 152 as shown in FIG. 1C, it is necessary to construct a three layer structure (a lower electrode, a mass portion, and an upper electrode) by forming the displacement detecting electrodes 155 and 157 in the upper and lower portion of the mass portion 152. However, since manufacturing a three layer structure is complex, a structure has been desired that would allow the Coriolis force to be generated in a plane direction of the substrate so as to simplify the manufacturing process while allowing Coriolis force detection.
Further, an angular velocity sensor 150 described above is manufactured by a semiconductor process technique for fine processing. However, in such a semiconductor process technique, the manufacturing process often results in the cross section of the beam 151 supporting the mass portion 152 becoming a trapezoidal shape having an oblique line direction formed by the advancing direction of the process, that is, the thickness direction of the beam is often not an ideal rectangle or square. This may be caused by for example, dispersion in the etching process or the like.
When the cross section of the beam 151 is trapezoidal, and the mass portion 152 is vibrated to the exciting direction, a leak vibrating component is generated in the thickness direction of the beam 151 when the beam 151 supporting the mass portion 152 vibrates in the exciting direction.
In a conventional angular velocity sensor 150, as mentioned above, the beam 151 supports the mass portion 152 in such a manner as to freely move to the exciting direction and to freely displace in the Coriolis force generating direction (that is, the thickness direction of the beam 151). The beam 151 further inhibits displacement to the vertical direction. Accordingly, most of the leak vibration component of the beam 151 due to the vibration to the exciting direction is generated to the Coriolis force detecting direction. Accordingly, by exciting the mass portion 152, even when the angular velocity is not applied, the mass portion 152 displaces to the detecting direction, that is, to the direction perpendicular to the drawing in FIG. 1A, so that an output is generated. Therefore, there is a possibility of deteriorating in detecting accuracy of the angular velocity sensor 150.
Further, in such a conventional angular velocity sensor 150, a space 154 is provided in a root area 159 disposed at one side of the beam 151 in the frame portion 160. In the space 154, the width in the z direction, that is, the width perpendicular to the exciting direction of the beam 151 is variably adjusted by a space width adjusting means (not shown). By adjusting the width of the space 154 using the space width adjusting means, the tensile stress of the beam 151 is changed, thereby adjusting the resonance frequency of the beam 151 so that the measurement in the best sensible state can be performed.
However, in such a conventional angular velocity sensor 150, since the beam 151 is used for both the exciting direction and the detecting direction, when the width of the space 154 is adjusted and the tensile stress is applied to the beam 151, the resonance frequency in the exciting direction and the detecting direction are increased. However, the resonance frequency to the exciting direction in the mass portion 152 and the resonance frequency to the Coriolis force detecting direction are not the difference .DELTA.f of the resonance frequency calculated by the characteristic of the angular velocity sensor. Accordingly, in the structure of adjusting the tensile stress of the beam 151 supporting the displacement of the mass portion 152 to the two directions mentioned above, there is a problem that adjustment for obtaining the best .DELTA.f is very difficult.
Further, in a conventional resonance type angular velocity sensor using the vibrator, since the component itself is large, a laser trimming process can be performed. However, as shown in FIGS. 1A, 1B, and 1C, in recent fine vibrating sensors manufactured by micro machining techniques using a material such as a silicon or the like, it is extremely difficult to fine adjust the frequency using a trimming process because the spot diameter of the laser beam is 10 or some tens of .mu.m larger than the area to be trimmed in the vibrating type sensor, which may be as small as a few .mu.m or less.
As mentioned above, conventional trimming methods of adjusting the resonance frequency of the vibrator to a constant value are not appropriate for adjusting the .DELTA.f. Accordingly, a simple structure for efficiently adjusting the .DELTA.f to the optimum value has been desired.